(define (make-center-percent c p)
  (make-interval (* c (- 1 p)) (* c (+ 1 p))))

(define (percent interval)
  (let ((width (/ (+ (upper-bound inverval) (lower-bound interval)) 2))
        (center (/ (- (upper-bound inverval) (lower-bound interval)) 2))))
  (/ width center))
